1. Field of the Invention
This invention relates to a method and apparatus for guiding endless woven fabric belts. Such fabric belts are used, for example, on Fourdrinier paper making machines. More specifically, this invention relates to such an apparatus which comprises a shower head and nozzles.
2. Description of the Prior Art
In such machines, paper is formed by first flowing a thin suspension of stock fibres in water from a head box slice onto the upper surface of a moving endless belt. As the belt then travels in contact with the table rolls or foils and suction boxes in the forming section of the machine, water is withdrawn from the stock through the belt, leaving a thin formation of self-supporting, matted fibres on the upper surface of the belt. The sheet of formed fibres is lifted off the belt at a couch roll at the downstream end of the forming section and the belt, after travelling around the couch roll, is returned through a series of return rolls to the upstream end of the machine where it travels around a breast roll and again passes under the slice and then to the forming section to complete the cycle which is continuous. The self-supporting matte of fibres, removed at the couch roll, passes to a press section where more water is removed by squeezing it at the nip of press rolls while sandwiched between layers of felt material also made up as endless belts. The remaining water is removed by passing the sheet of matted fibres over steam heated rolls under endless dryer felts in the dryer section of the machine.
The endless belts employed in the forming section of the paper machine are generally woven from synthetic monofilament or multifilament yarns or from metal strands such as bronze or stainless steel or from a combination of these materials. Belts employed in the press section are generally felted material having a base fabric of woven or non-woven natural or synthetic fibre yarns. The dryer belts are generally made of relatively heavy fabric woven of natural or synthetic fiber yarns containing fibres of heat resisting mineral. In any case, a belt runs like a conveyor belt over machine components and is subject to stress variations which can upset stability to cause it to run spirally towards one or the other side of the machine. It may also happen that a woven belt of any of these types has a built-in bias which tends to make it run consistently towards one side of the machine. It is therefore necessary to provide a means to continuously guide the belt and keep it centered.
It has usually been the practice to keep the belt running truly in the center of the machine by means of a guide roll over which the belt runs in the return section of the machine. The guide roll can be moved at one end transversely to its axis and in the plane of the machine to steer the belt and tend to keep it on its course. The end of the roll is usually moved by a pneumatic, hydraulic or mechanically driven system which is controlled by a sensing device at one edge of the belt. In operation, when the belt runs towards one side of the machine the edge of the belt activates the sensing device which in turn activates the drive mechanism to offset the end of the guide roll in such a way that the belt is steered towards the opposite side of the machine.
A disadvantage of the known method of guiding the belt, particularly on the slower operating machines, is that the reaction to the steering effect of the guide roll is delayed because the displacement of the end of the guide roll is necessarily quite small and the belt must make many revolutions on the machine before any appreciable amount of belt displacement is realized. Thus, in the case of belts that are accidentally influenced to run off one side of the machine, the corrective action may come too late to be effective in preventing damage to the belt.
A further disadvantage, particularly in the case of wide, high speed paper machines, is that the guide roll, like other rolls in the system, must be large in diameter to counteract deflection, and the mechanism to effect displacement of one end of the roll must be correspondingly large and powerful thus making it expensive to provide and maintain.
It is also known to use showers in association with paper making machines. Although the showers are normally used for the purpose of cleansing the endless belt fabric, U.S. Pat. No. 3,830,691, Truesdale et al., issued Aug. 20, 1974 teaches a specialized use for such a shower. Specifically, the shower is employed in the Truesdale et al. patent for the purpose of spreading the fabric in a low tension part of the run.